321
Configuring STP
Information About Configuring STP
Spanning Tree and Redundant Connectivity
You can create a redundant backbone with spanning tree by connecting two switch interfaces to another device or to
two different devices, as shown in Figure 38 on page 321. Spanning tree automatically disables one interface but
enables it if the other one fails. If one link is high-speed and the other is low-speed, the low-speed link is always
disabled. If the speeds are the same, the port priority and port ID are added together, and spanning tree disables the link
with the lowest value.
Figure 38 Spanning Tree and Redundant Connectivity
You can also create redundant links between switches by using EtherChannel groups. For more information, see
Configuring EtherChannels, page 1027
Spanning-Tree Address Management
IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to be used by
different bridge protocols. These addresses are static addresses that cannot be removed.
Regardless of the spanning-tree state, each switch receives but does not forward packets destined for addresses
between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the CPU on the switch receives packets destined for 0x0180C2000000 and
0x0180C2000010. If spanning tree is disabled, the switch forwards those packets as unknown multicast addresses.
Accelerated Aging to Retain Connectivity
The default for aging dynamic addresses is 5 minutes, the default setting of the mac address-table aging-time global
configuration command. However, a spanning-tree reconfiguration can cause many station locations to change. Because
these stations could be unreachable for 5 minutes or more during a reconfiguration, the address-aging time is
accelerated so that station addresses can be dropped from the address table and then relearned. The accelerated aging
is the same as the forward-delay parameter value (spanning-tree vlan vlan-id forward-time seconds global
configuration command) when the spanning tree reconfigures.
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN basis. A
spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that VLAN to be subject to
accelerated aging. Dynamic addresses on other VLANs can be unaffected and remain subject to the aging interval
entered for the switch.
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